Move the release version string to version.mk.

Previously we defined the release version string in configure.ac,
and overrode that with 'git describe --tags' if possible. This
made it difficult for non-autoconf builds to set their version
string correctly.

Instead we create, and check into version control, a file called
version.mk which defines OPUS_VERSION. The configure script reads
that file and uses it as a fallback if the git revision isn't available.

The expectation is that version.mk will be manually updated for
releases, just as the previous configure.ac version was. However,
since this is a simpler format, it is easier for alternate build
systems to use, reducing the number of places which must be updated.

Also removes the OPUS_MINOR_VERSION, etc. defines from config.h.